The buried wiring structure is formed by burying a wiring material in a wiring opening such as a wiring trench or hole formed in an insulating film with a wiring formation technique referred to as Damascene technique (Single-Damascene technique and Dual-Damascene technique). However, when a material of the main wiring is copper (Cu), Cu tends to be diffused into an insulating film compared with a metal such as aluminum (Al). For this reason, in order to prevent the buried wiring made of copper from directly making contact with the insulating film, the surface (bottom and side surfaces) of the buried wiring is covered with a thin barrier metal film, thereby suppressing or preventing copper in the buried wiring from being diffused into the insulating film. Also, a barrier insulating film as a wiring cap made of, for example, a silicon nitride film is formed on an upper surface of the insulating film having a wiring opening formed therein to cover the upper surface of the buried wiring, thereby suppressing or preventing copper in the buried wiring from being diffused from the upper surface of the buried wiring into the insulating film.
In recent years, intervals between the buried wirings have been decreased with the increase in integration degree of a semiconductor device. This increases parasite capacitance between wirings to cause a signal delay, so that cross talk occurs between adjacent wirings. For this reason, it is desired to reduce the parasite capacitance between wirings. For the reduction of the parasite capacitance between wirings, a low dielectric-constant material is used for an inter-wiring insulating film. Meanwhile, for example, Japanese Patent Application Laid-Open Publication No. 2003-297918 (Patent Document 1) discloses a technique of forming each wiring in a tapered shape and also forming an air gap between these wirings. By means of this air gap, inter-wiring capacitance is reduced. Also, in Japanese Patent Application Laid-Open Publication No. 2006-120988 (Patent Document 2), the inter-layer insulating film is etched deeper than the bottom of the wirings to further reduce the capacitance.